1. Field
Embodiments of the invention relate to the field of analog and digital copy protection; and more specifically, to the field of copy protection of content stored on digital recordable media.
2. Description of the Related Art
Various well known copy protection schemes for video signals include those disclosed in U.S. Pat. No. 4,631,603, by John O. Ryan, Dec. 23, 1986 and assigned to Macrovision Corporation, directed to modifying an analog video signal to inhibit making of acceptable video recordings therefrom. The '603 patent discloses adding a plurality of pulse pairs to the otherwise unused lines of a video signal vertical blanking interval, each pulse pair being a negative-going pulse followed closely by a positive-going pulse. The effect is to confuse AGC (automatic gain control) circuitry of a VCR (video cassette recorder) recording such a signal, so that the recorded signal is un-viewable due to the presence of an excessively dark picture when the recorded signal is played back.
Another analog video protection scheme is disclosed in U.S. Pat. No. 4,914,694 issued Apr. 3, 1990, to Leonard, and assigned to Eidak Corporation: The Eidak system (see Eidak Abstract) increases or decreases the length of each video field from the standard length, either by changing the time duration of the respective horizontal line intervals in each field while keeping a constant, standard number of lines per frame, or by changing the number of horizontal line intervals which constitute a frame while maintaining the standard duration of each line interval.
These video protection systems modify the video signal to be recorded (for example, on tape, magnetic disk, optical disk, or other recordable media) or to be broadcast (for example, protected pay-per-view (PPV) television programs) and to make viewable copying by ordinary VCRs or other recordable media difficult or impossible. When a video tape, or the like, on which is recorded the copy protected video signal is played back for viewing using a VCR or similar playback device, the copy protection process is essentially transparent, i.e., it does not interfere with viewing the originally recorded content. However, any attempt made to copy the video signal from the tape using a second VCR to record the output of the first (playback) VCR yields a picture degraded to some extent, depending on the efficacy of the particular copy protection system. These conventional video copy protection systems protect only analog video signals.
Also well known are digital video recorders, which both record and play back digitally. The advantage to the user of a digital recorder is that so long as the signals are recorded and played back in the digital domain, each successive generation of copies is without any significant reduction in picture quality, unlike the case with conventional analog recording technology.
Many consumer products today include both digital and analog inputs and outputs. Some of these systems have the capability to record and playback digital signals, while still having analog output capabilities. Thus, these systems have the capability internally to convert input analog signals into digital signals, and play back the digital signals as an analog video stream from a digital storage device. A digital versatile disk (DVD) player is one example of such a consumer device that retains digital video signals and can output a corresponding analog video stream. During playback, the digital data stream from an optical disk, for example, may be available both as a digital signal for display by a digital television set or converted within the device to a conventional analog video output signal (such as the NTSC signal used in the United States, or PAL or SECAM used elsewhere).
Because digital video systems are capable of high fidelity reproduction, which in turn facilitates high quality copying, it is important that such devices for consumer use be designed to inhibit or discourage unauthorized recording. For instance, it is important to prevent use of recorders for illegally duplicating copyrighted video material, and also to prevent playing back of such illegally duplicated material.
U.S. Pat. No. 5,315,448 (the '448 patent), by John O. Ryan, describes a hybrid digital and analog recorder that records digitally and provides copy protection in both the digital and analog domains. For externally supplied analog video, the recorder detects the presence of copy protection and in response disables recording. For externally supplied digital video, both anti-copy bits and serial copy protection bits are detected to respectively (1) disable recording and (2) prevent later copying by a second digital recorder. For playing back of recorded material, the presence of anti-copy bits is detected in the digital playback video, and the digital playback video upon being converted to an analog signal is modified by an analog video copy protection process. In another embodiment of the '448 invention, analog or digital source video material (either prerecorded or from an external source) is provided with a copy protection flag or trigger. Detection of the flag by a playback device results in modification of the played back standard video signal with an analog copy protection process. This embodiment is suitable for playback devices where the source video cannot be copy protected, but a standard (NTSC) video signal is provided from a played-back recording.
The analog video copy protection process described in the '448 patent includes the use of an ACP (anti-copy process) signal generator, such as an embodiment described in U.S. Pat. No. 4,631,603. This ACP signal generator generates an analog video anti-copy signal. The ACP signal generator then adds this analog anti-copy signal to the output signal of a digital to analog converter, which has converted the digital video output signal from a DVD, for example, to an analog (for example, NTSC) signal. Alternatively, the ACP signal generator can be implemented as shown in above-referenced U.S. Pat. No. 4,914,694 for modifying the “TV signal source”. It will be apparent to those of ordinary skill in the art that other ACP signal generator implementations can be used. Thus, using the '448 invention, the analog (NTSC) video signal presented at an analog output terminal is an analog video signal modified by the analog anti-copy process. This prevents the making of acceptable video recordings on existing analog recording devices from the signal provided from the pre-recorded video signal.
Although the '448 patent describes an effective analog copy protection system, it would be beneficial to separate out the particular analog anti-copy process from the hardware that actually modifies the analog video signal for output to a rendering or recording device. By separating the anti-copy process from the signal modification hardware, the system would achieve a level of flexibility and configurability not present in systems today.